1. Field of the Invention
This invention relates to an apparatus for forming a paperboard receptacle from a paperboard blank in male and female dies. More particularly, the present invention relates to injecting steam into one of the forming dies against a surface of the blank during drawing of the paperboard blank into a receptacle.
2. Description of the Prior Art
Paperboard receptacles are formed from blanks comprising a laminate of paperboard and a plastic film (e.g., a polyester such as polyethylene terephalate). The portions of the blanks which will form the corners of the receptacle are scored to facilitate forming of the receptacle corners.
Conventional systems for forming a paperboard receptacle have used mating male and female dies to form the receptacle. The peripheral portions of the dies engage the periphery of the blank to place the blank under tension during the drawing process. The central portion of the blank is engaged on its opposite surfaces by mating movable portions of the male and female dies. The die surfaces may be coated with polytetrafluoroethylene to enhance the release of te formed receptacle from the dies. However, such coatings have been ineffective for forming receptacles at relatively high temperatures (i.e., above 180.degree. F.) since the formed receptacle tends to stick to the female die.
The primary problem of conventional systems for forming paperboard receptacles involves tearing or rupture of the paperboard at the sharp bends between the bottom and side walls of the receptacle. These tears and ruptures can migrate to the scored areas at the corners where the blank is placed under maximum stress during the drawing process and where the blank has been weakened by the scoring. In conventional systems, the tearing and rupturing tendency is increased because the paperboard is not adequately plasticized to permit articles to be readily formed with sharp bends and corners without tearing, rupturing or cracking. Additionally, the friction or adhesion between the paperboard and forming dies causes a discontinuous, creeping drawing action, rather than a smooth, continuous drawing action. The discontinuous, creeping action tends to form wrinkles and will not form smooth side wall surfaces.
Another problem associated with the use of conventional systems for forming paperboard receptacles involves separation of the plastic coating from the underlying paperboard. In conventional systems, the paparboard is passed through a water bath immediately prior to forming to wet the paperboard. This additional wetting of the paperboard, providing a moisture content in the paperboard is of at least ten weight percent, to improve drawing causes wet adhesion problems which decrease the adhesion of the plastic film to the paperboard. The pre-forming wetting operation of conventional systems is also disadvantageous in that it increases the dwell time required in the forming cycle in the dies, may necessitate an additional drying cycle and otherwise adversely affects the finished paperboard receptacle, particularly the printing thereof.